Dropping an Einstein thought experiment down an elevator shaft

Researchers create a compact device that can produce and monitor a Bose …

Einstein was famous for performing what are termed "thought experiments"—hypothetical situations that illustrate the consequences of a theory—that allowed him to gain insights into the natural world without bothering to leave the confines of his own brain. One of these experiments involved placing a subject in an elevator that is then allowed to free fall. As far as Einstein could tell, there would be no way for the subject to tell if she was falling into the local gravity well, or simply out in space, free from gravity's influence—this insight supposedly helped him formulate his theory of relativity.

But it turns out that this is the sort of thought experiment that might be useful to translate to reality, since building a laboratory equivalent of an elevator shaft is a whole lot easier than sending something into space. But the sorts of physics experiments we'd like to do tend to involve large, complex, and delicate equipment that wouldn't take well to being dropped down an elevator shaft. In an impressive bit of engineering, researchers in Germany have created a device that produces and monitors a Bose-Einstein condensate while being dropped down a 146 meter high shaft.

The drop shaft, located at the Center of Applied Space Technology and Microgravity in Bremen, is pictured at right in all its phallic glory. The sample area is magnetically shielded and can have the air evacuated. Samples dropped from the top will experience nearly five seconds at 10-6g before experiencing a cushy landing in an eight meter deep pool of loose polystyrene packing foam. There's also a catapult on the bottom for launching samples that can handle acceleration. During their trip to the top of the shaft and back, these samples will get about nine seconds of microgravity.

But the facility is only half of the story. Bose-Einstein condensates involve getting a large collection of bosons—in this case, rubidium atoms—into an identical energy state. Once this is achieved, the collection of atoms acts as a single entity, described by a single wave function—in effect, a quantum mechanical system on macroscopic scales. Unfortunately, getting a collection of atoms in a single state is only practical by putting them in a very low energy state, which means cooling them to near absolute zero. That, in turn, requires what's typically a large collection of lasers to cool them, and then an additional collection of hardware to monitor the Bose-Einstein condensate's behavior.

The stunning bit of engineering here is that the research team crammed all of the requisite hardware into a 215cm high cylinder with a diameter of 82cm. They were aided immensely by the development of something called an "atom chip" that has nothing to do with Intel—instead, it's a solid state device that can spit out Bose-Einstein condensates with a minimum of fuss. In the authors' device, these were held to a very chilly nine nanoKelvin.

Once the condensates were in place, the device was set loose and, after a second or so, the system equilibrated to its microgravity state, after which the evolution of the quantum system was followed. It turned out to be exquisitely sensitive to local magnetic fields generated by the apparatus itself and the vacuum pump used by the facility. Once those factors were accounted for, the behavior of the Bose-Einstein condensate was identical to that predicted by theory within the limits of the monitoring equipment.

Although nothing of the sort appeared in this initial description of the system, one of the goals of this work is to establish a system in which we can test the relativistic behavior of a quantum system, and possibly start to identify where the boundaries between these two domains reside. One of the more interesting effects that could be amenable to study is frame dragging, in which a massive rotating object gives the space-time in its vicinity a bit of a swirl. The work involved in creating a compact Bose-Einstein system may also lay the foundation for sending similar hardware into space, where tests can be performed in a stable microgravity environment.